6

Chiari malformations include a group of complex anomalies of the hindbrain with different etiology, pathophysiology, and clinical feature. These malformations range from the simpler to the more complex varieties of presentation, signifying their stages of appearance during embryologic differentiation and development (Table 6.1). Various management modalities have been put forth to treat this entity. Over the years there has been a significant advance in understanding the pathophysiology, diagnostic modalities, and management strategies.

It was John Cleland (1835–1925), a Scottish anatomist, who first described a case of hindbrain malformations in 1883.¹ In his article, “Contribution to the Study of Spina Bifida, Encephalocele, and Anencephalus,” Cleland described the pathophysiologic findings in nine infants at autopsy and two chick embryos.¹ Later Hans Chiari (1851–1916), an Austrian pathologist who practiced medicine in Vienna, Prague, and Strasbourg, published his work in 1891.^2,3 Chiari described the type I malformation as “peg-like elongation of tonsils and medial divisions of the inferior lobes of the cerebellum into cone-shaped projections which accompany the medulla oblongata into the spinal canal,” while sparing the medulla.² In 1894, Julius Arnold (1835–1915), who studied under Virchow and Friedreich, described a case of an infant with spina bifida and elongation and descent of the inferior part of the cerebellum into the spinal canal.^2,4 In 1907, Schwalbe and Gredig, while working in Arnold’s laboratory at Heidelberg, described four cases with meningomyelocele and alterations in the brainstem and cerebellum, and used the term “Arnold-Chiari.”^1,3,4

With the advent of magnetic resonance imaging (MRI), the intracranial anomalies associated with various Chiari malformations have become much clearer, and MRI became the imaging modality of choice in the diagnosis of Chiari malformations and the associated syrinx. MRI had been used to quantify the extent of tonsils below the foramen magnum. Aboulezz et al⁵ quantified the extension of tonsils below the foramen magnum based on MRI findings. The extension of tonsil below the foramen magnum is considered normal if it is less than 3 mm, borderline if it is between 3 and 5 mm, and clearly pathologic when it exceeds 5 mm.⁵ According to ]Barkovich et al,⁶ an MRI demonstration of tonsillar ectopia of <2 mm is probably of no clinical significance in the absence of syringomyelia. Further studies have shown that a small posterior fossa predisposes to hindbrain overcrowding, especially in patients with Chiari type I malformations.⁷ Further advances in MRI techniques, such as cine-MRI, greatly helped to study the alterations in the cerebrospinal fluid (CSF) flow at the foramen magnum⁷ and became a useful tool to evaluate the results of posterior fossa decompression radiologically.

♦ Pathogenesis of Chiari Malformations

Developmental Arrest

The embryologic timing of the occurrence events responsible for the development of the Chiari malformation is not very clear. Daniel and Strich⁸ suggested that the abnormalities might develop early in the embryonic life. Based on their observation, they postulated that failure to form pontine flexure at the 6th week of fetal life may be one of the factors accounting for the marked elongation of the brainstem. They observed that in the initial part of fetal life (about 37 days), the length of hindbrain relative to forebrain is very great, and only after the formation of the pontine flexure does the hindbrain shorten. They also postulated the mechanism for the development of the kink. The cervical flexure is produced in the fourth week of fetal life and undone during the second month.⁸ If there is an elongated hindbrain during this process of straightening out the cervical flexure, it may result in a possible kink. The caudal displacement of the cerebellum and brainstem would result in an enlarged foramen magnum and a smaller posterior fossa that permits the tentorium to have a low insertion. However, this theory fails to explain the other associated spinal and cranial abnormalities.

Table 6.1 Description of Chiari Malformations

The study by Marin-Padilla and Marin-Padilla9 showed that administration of vitamin A to pregnant hamsters induces types I and II Arnold-Chiari malformations. According to this theory, the primary defect is mesodermal, involving the cranial base. This has resulted in a small and short posterior fossa that is inadequate to contain the developing neural structures of the region.⁹ The primary paraxial mesodermal defect can affect the embryo at any time relative to closure of the neural folds.⁹ According to Sarnat,¹⁰ the crowding theory postulated by Marin-Padilla and Marin-Padilla would only impart a significant influence in late gestation. Nishikawa et al¹¹ quantitatively measured the volume of the posterior cranial fossa and volume of the neural structures in the posterior cranial fossa. Their results strongly suggest the overcrowding hypothesis. Overcrowding of the neural structures (normal-sized hindbrain) in the underdeveloped posterior cranial fossa induces a downward herniation of the hindbrain.

Hydrodynamic Mechanisms

According to Gardner,¹² a balance exists between the pulsatile flow in the lateral ventricle choroid plexus and the fourth ventricle choroid plexus. The final position of tentorium is determined by the hydrodynamic effects of the anterior and posterior choroid plexus. According to Gardner, if the caudal end of the neural tube ruptures, the competing effect of the posterior choroid plexus is largely lost. The relatively increased effectiveness of the anterior choroid plexus causes the lateral ventricle to overdistend.

Chiari malformations result from imbalances between the pulsating choroid plexus in the fourth ventricle and in the lateral ventricle. Overactive supratentorial pulsations might cause tentorial migration, resulting in the development of a Chiari malformation.¹² This theory also fails to explain the occurrence of the other associated anomalies. The hydrodynamic theory does not explain the associated supratentorial anomalies.

Oligo–Cerebrospinal Fluid or the Unified Theory

This theory was proposed by McLone and Knepper.¹³ According to this theory, distention of the embryonic ventricular system is critical to normal development of the brain. If during the early part of the fetal development the presence of an open neural tube defect leads to escape of the CSF and fails to distend the embryonic ventricular system, the small posterior fossa is a consequence.

Neuroschisis

According to Padget,¹⁴ splitting open the neural tubes results in escape of the fluid from the neural tube and the formation of “neuroschistic blebs.” These blebs might heal totally, lead to formation of a spina bifida occulta, or might rupture with the eversion of neural cleft margins, leading to an open spinal lesion. Padget explained that a Chiari malformation is a sequel to a neural cleft, which allows fluid to escape from the neural tube. Embryonic microcephaly results, and the cerebellar primordia prematurely approximate and fuse in the posterior fossa, which is already small. Further development of the cerebellum in a small posterior fossa results in herniation out of the foramen magnum with subsequent obstruction of the fourth ventricle, producing hydrocephalus, whereas the microcephaly leads to folding and fusion at the level of the midbrain, resulting in aqueduct stenosis.

Pulsion Theory

According to this theory, fetal hydrocephalus pushes the contents of the posterior fossa downward from above.^12,15–17 This theory was originally proposed by Chiari.¹⁰ This theory cannot explain the occurrence of the Chiari malformations without hydrocephalus and other associated anomalies.

Neuroectodermal-Mesodermal Spatial Dyssynchrony

Jennings et al¹⁸ hypothesized that the etiologic event responsible for the Arnold-Chiari malformation is the caudal displacement of the site of the initial fusion of the neural folds. They postulated that the normal zone of fusion at the third and fourth somites is displaced caudally below the third to fifth somite pairs, thus displacing the area of formation of the cervicomedullary junction. Their observations were based on the study of a 130-day-old human fetus with associated Arnold-Chiari malformation and thoracolumbar myeloschisis, which revealed evidence of neuroectodermal mesodermal spatial dyssynchrony.

This theory, proposed by Penfield and Coburn19 in 1938, states that traction due to tethering of the spinal cord at a lower level prevents upward migration during early development, while the cerebellum and brainstem are pulled down as the vertebral column grows. Goldstein and Kepes²⁰ tested the effect of spinal cord tethering on the development of Arnold-Chiari malformation in experimental animals and could not find any evidence of the malformation in the experimental animals that survived and reached adulthood. It is very unlikely that traction forces affect the cervical spinal cord and medulla.²⁰ Traction theory fails to explain the lack of other observations, such as the medullary kink, and the spinal cord is not always tethered in patients with Chiari malformations.

Molecular Genetic Hypothesis

Sarnat^21,22 postulated that Chiari malformation II is a disturbance of rhombomeric segmentation and ectopic expression in the embryonic hindbrain due to genetic mutation. HOX family genes in particular are implicated because they not only program hindbrain segmentation but also are important in the development of basioccipital, exoccipital, and supraoccipital bones of paraxial mesoderm origin.²³ The same group studied several markers like vimentin in the ependyma of patients with Chiari II malformations.¹⁰ The results of the study showed that vimentin is focally upregulated only in areas of dysgenesis. Based on their observation, they speculated that the focal upregulation of vimentin could be a secondary event as a result of defective expression of another gene. This observation may also support a molecular genetic hypothesis.

♦ Classification of Chiari Malformations

Chiari 0

Iskandar et al²⁴ in 1998 reported resolution of syringohydromyelia without hindbrain herniation in five patients after posterior fossa decompression. They referred to such cases of syringomyelia without hindbrain herniation as the “Chiari 0” malformation. Later the same researchers showed that the contents of the posterior fossa are compromised and distorted in these patients with syringohydromyelia without hindbrain herniation.²⁵ They suggest that a smaller than normal posterior fossa is just enough to compromise CSF egress out of the cranium.²⁵

Milhorat²⁵ feels that cerebellar tonsils are paramedian structures, and on midsagittal MRI scans minimal tonsillar herniation might be missed. Milhorat et al,⁷ in their study of 364 patients with Chiari I malformation, showed that in these patients the CSF volume in the posterior fossa is significantly smaller, and tonsillar herniation of less than 5 mm does not exclude the diagnosis of Chiari I malformation. Milhorat²⁵ suggests that a better description of such cases with “Chiari 0” malformation would be “borderline Chiari malformations.” Rather than classification issues, one may have to investigate these patients without significant tonsillar herniations using cine-MRI⁷ scans to document disturbance in the CSF circulation in the posterior fossa before subjecting them to hindbrain decompression.

Chiari I

Chiari I malformation is characterized by caudal displacement of the cerebellar tonsils by more than 5 mm into the cervical spinal canal below the plane of the foramen magnum. According to Milhorat et al,⁷ tonsillar herniation of less than 5 mm does not exclude the diagnosis of Chiari I malformation. This is the most commonly seen clinical entity and is described as synonymous with tonsillar herniation. It is usually referred to as adult-type Chiari malformation due to its incidence in the second or third decade of life.⁷ The exact incidence of Chiari I malformations is not known. Meadows et al²⁶ reviewed 22,591 patients who underwent MRI of the head and cervical spine. Of these only 175 patients (0.8%) had Chiari I malformation.

Associated Anomalies

Craniovertebral anomalies are frequently seen in patients with Chiari malformation.^27,28 In the study by Milhorat et al,⁷ the most common bony anomalies were reduced height of the supraocciput (84%), increased slope of the tentorium (82%), reduced length of the clivus (49%), retroflexion of the odontoid (26%), and basilar invagination (12%). Tubbs et al²⁹ graded the posterior angulation of the odontoid process in patients with Chiari I malformation and found that higher grades of posterior angulation are more often associated with syringomyelia. Other commonly seen osseous abnormalities are platybasia, midline occipital keel, remnants of proatlas, atlantoaxial dislocations, Klippel-Feil anomaly, empty sella, and clival concavity.^30,31

The incidence of syringomyelia associated with Chiari malformation varies between 50% and 75%.^7,32–35 The common location of the syrinx is cervical followed by the thoracic spinal cord (Fig. 6.1),³⁵ occasionally with holocord syrinx (Fig. 6.2). A thickened dural band posterior to the arch of C1 may be frequently encountered during surgery.³⁵ Chiari I malformation is associated with hydrocephalus in 7 to 10% of cases.^7,35 An occasional patient might show an elongated brainstem with medullary kinking (Fig. 6.3).

Chiari 1.5

This group comprises patients with tonsillar ectopia as seen in Chiari I malformation. In addition, they exhibit caudal descent of the brainstem (Fig. 6.4). This has been specifically referred to as “Chiari 1.5”^36,37. No other sign or symptom was peculiar for Chiari 1.5 malformations.³⁷

Myelomeningocele (MMC) is nearly always associated with Chiari II malformation.38 Luckenschadel skull is an ossification disorder in which the fetal skull appears fenestrated. It is almost always associated with Chiari II malformation and MMC.³⁹ Other anomalies associated with Chiari II malformation are petrous scalloping, enlargement of the foramen magnum, fenestrations of the falx, falx hypoplasia, and hypoplastic tentorium with wide incisura.⁴⁰ The inion is lower, and there may be an occipital keel with less frequency of basilar invagination. The upper cervical spine shows Klippel-Feil anomaly with hypoplastic posterior arch of C1 and scalloped dens.⁴¹

Other abnormalities associated with Chiari II malformation, as detected in MRI scans, are hypothalamic adhesions (48.6%), low anterior commissure (38%), abnormalities of the corpus callosum and hippocampal commissure (57%), callosal ridge (60%), cortical posterior stenogyria (72%), gray matter heterotopias (19%), hippocampal abnormalities (85%), and atypical sulcation of the adjacent temporomesial cortex (93%).³⁸ The midbrain is elongated with a shortened quadrigeminal plate. Peaking is seen with fusion of the colliculi (tectal beaking). Callen et al⁴² observed tectal beaking in 66% of patients with Chiari II malformation on prenatal sonography. The caudal displacement of the lower brainstem may create a medullary kink in the cervical canal.⁴³

Hydrocephalus is seen in 90% of patients with Chiari II malformation.⁴⁴ The other ventricular abnormalities found in patients with Chiari malformation II are “shark-tooth deformity” of the third ventricle, colpocephaly, beaking of the frontal horns, severely deformed lateral ventricles (although they may also be normal), small fourth ventricle, and small aqueduct.⁴⁵ The vast majority of type II patients exhibit syringohydromyelia, sometimes with exophytic components mimicking arachnoid cyst^36,46 (Fig. 6.5). In approximately 6 to 15% of cases, there is an associated spilt-cord malformation.^47,48

Fig. 6.2 A cervicothoracic syrinx seen in a Chiari malformation in this sagittal T1-weighted MRI. Most often this type of syrinx disappears after decompression at the foramen magnum.

Type III is the rarest and most severe form of all the Chiari malformations. It is characterized by occipital or cervical encephalocele along with the above intracranial abnormalities seen with type II malformation.49,50 The sac of encephalocele contains dysmorphic and ischemic neural elements (Fig. 6.6).

Other cranial abnormalities, such as small posterior fossa, low tentorial attachment, scalloping of the clivus, tectal beaking, and dysgenesis of severely deformed corpus callosum, are seen.49

Chiari IV

This type is characterized by marked cerebellar hypoplasia or aplasia and tentorial hypoplasia. There is no hindbrain herniation.

♦ Clinical Presentation

Of all the Chiari malformations, type I is most commonly encountered. After the advent of MRI scans, several atypical, silent, or incidental entities have been reported. At Louisiana State University Health Sciences Center in Shreveport, Louisiana, the experience with these malformations amounts to 95 patients, of whom approximately 20% were children, although 9.5% presented after 55 years of age. There was female preponderance: 72% were females and 28% were males.

Type I Malformations

The signs and symptoms in patients with Chiari I malformation are generally related to the compression of the neural structures by the herniated tonsils⁵¹ and by the presence of an associated syrinx. Clinical manifestations can be grouped into cranial nerve dysfunction, long tract deficits, and cerebellar dysfunction.

Fig. 6.5 A more severe descent of the cerebellum along with a low-lying fourth ventricle is seen in this T1-weighted MRI. Note the syringomyelia starting right at the level of spinal cord compression by the tonsils. The brainstem is showing kink with compression of medulla.

Suboccipital headache is one of the common presentations of the Chiari I malformation. These headaches are usually exaggerated by physical exertion and the Valsalva maneuver.⁷ Ocular disturbances such as retro-orbital pain, floaters or flashing lights, blurred vision, photophobia, and diplopia are frequently reported by patients.⁷ Otoneurologic disturbances such as dizziness, disequilibrium, tinnitus, decreased hearing or hyperacusis, vertigo, and oscillopsia are commonly seen.^7,52 Downbeat nystagmus is characterized by slow upward drift and fast downward phases.⁵³ One of the common causes for downbeat nystagmus is Chiari malformation.⁵³

Lower cranial nerve, brainstem, and cerebellar disturbances are seen in up to 52% of patients.⁷ The most common symptoms are dysphagia, sleep apnea, dysarthria, and incordination.⁷ Objective signs are absence or impairment of the gag reflex, vocal cord paralysis, facial hypoesthesia, dysmetria, and truncal ataxia. Long tract signs include posterior column dysfunction and pyramidal weakness.

Patients might present with clinical features due to an associated syrinx. The signs and symptoms vary with the location and extent of the syrinx. The most common symptoms are muscular weakness, paresthesias, dysesthesia, nonradicular segmental pain, analgesia, spasticity, trophic phenomenon, and poor position sense.7 Dissociated sensory loss entails loss of pain and temperature sensation with preservation of light touch. An expanding syrinx disrupts the decussating sensory fibers passing from the dorsal horn to the opposite lateral spinothalamic tract. Disassociated sensory loss is not always a necessary finding for the diagnosis of syringomyelia.⁵⁴ These patients might have features of both upper motor neuron and lower motor neuron dysfunction.

Fig. 6.6 (A–C) An occipital encephalocele in a young child. Intraoperative pictures show the atrophic neural elements inside the meningeal sac and after excision.

The presenting features in children sometimes differ from the presenting features in adulthood. Children under 3 years of age might present with oropharyngeal dysfunction, and slightly older children (3 to 5 years of age) might present with scoliosis, headache, or neck pain.⁵⁵

Type II Malformations

The first sign in these patients would be an MMC, taking into account that Chiari II malformation is universally associated with MMC.⁴⁵ Infants and children younger than 2 years of age present most frequently with cranial nerve and brainstem dysfunction, and the most common and potentially fatal symptom in these age groups involves respiratory difficulties.45 Other symptoms in infants include hypotonia, opisthotonus, nystagmus, weak cry, and developmental delay.⁴⁵ Symptoms in older children are not life threatening and tend to be more insidious, and features of cervical myelopathy are the hallmark finding in these age groups.⁴⁵ Older children can also present with ataxia, occipital headache, neck pain, and features of syringomyelia⁴⁵

Type III Malformations

These children usually present with breathing difficulty, swallowing problems, seizures, developmental delay, weakness and hyporeflexia in the upper extremities, and spasticity in the lower extremities.^56,57

♦ Diagnostic Imaging

Magnetic resonance imaging is the diagnostic test of choice. It provides simultaneous visualization of the cervical spine and the entire neuraxis. Syringomyelia and syringobulbia are easily demonstrated. MRI facilitates the identification of the venous anomalies of the posterior fossa. The level of the torcular and the transverse sinus are important to plan the posterior fossa decompression. In cases of Chiari III malformation, MRI enables the surgeon to identify the amount of neural tissue in the encephalocele and the position of the brainstem. The anomalies associated with various Chiari malformations were described above.

Dynamic studies are useful to demonstrate CSF flow around the foramen magnum, and can show obstruction by hindbrain structures or evidence of syringomyelia. Cine-MRI is helpful in demonstrating a CSF flow disturbance at the foramen magnum.⁷ McGirt et al⁵⁸ demonstrated that normal preoperative hindbrain CSF flow is an independent risk factor for treatment failure after decompression for Chiari I malformation regardless of the degree of tonsillar ectopia. In another study the presence of decreased CSF flow both ventral and dorsal to the cervicomedullary region was associated with improved response to posterior fossa decompression.⁵⁹ Flow studies and cine-mode MRI are useful for postoperative evaluation following decompressive surgery. Flow obstruction, decreased velocities, and a shorter period of caudal CSF flow in the foramen Magendie and foramen magnum in patients with more than 5 mm of tonsillar herniation are reported.⁶⁰

♦ Brainstem Auditory Evoked Potentials

Zamel et al⁶¹ studied the role of brainstem auditory evoked potential (BAEP) monitoring during posterior fossa decompression. They showed that a predominant improvement in central conduction in most of their patients occurred during the period of bony decompression. Similarly, Anderson et al⁶² found that improvement in conduction velocity occurs after bone decompression and division of the dural band. The predictive value of the improvement in BAEP needs to be assessed further in larger studies to establish its role in the surgical management of Chiari malformations.

♦ Surgical Treatment and Outcome

Chiari I Malformations

With the advent of MRI an increasing number of patients are being diagnosed with asymptomatic Chiari I malformation. An important controversy in the management of asymptomatic Chiari I malformation concerns the role of prophylactic surgery in asymptomatic patients. Novegno et al⁵¹ studied the natural history of Chiari I malformation. Their findings suggest that a conservative approach is indicated in both asymptomatic and slightly symptomatic patients, with periodic follow-up. Haroun et al⁶³ reported that in their survey of the members of the pediatric section of the American Association of Neurological Surgeons, the majority of the respondents rejected the routine use of prophylactic surgery for Chiari I malformation.

Cerebrospinal Fluid Diversion Procedures

In patients with hydrocephalus, the CSF diversion procedure is considered the best initial treatment option. Ventriculoperitoneal (VP) shunting is the most common CSF diversion procedure. Recently, endoscopic third ventriculostomy has been shown to be efficacious in patients with hydrocephalus associated with Chiari I malformation.⁶⁴ Following the CSF diversion procedure, if the symptoms persist or if the syrinx does not show improvement or enlarges, then one needs to direct one’s attention to a Chiari decompression.

Posterior Fossa Decompression

♦ Surgical Technique

In brief, this operative procedure is performed with the patient in the prone position. A midline skin incision is made from 2 cm above the inion up to C2. The incision is deepened down to the bone along the midline avascular plane. Self-retaining retractors are placed, and the musculature is gently retracted. At the level of the superior nuchal line, the suboccipital muscles are divided with a thin rim attached to the bone. The muscles are separated subperiosteally from the underlying occipital squamous bone. Inferiorly the muscular attachment to the retractor arch of the atlas is detached by sharp dissection. Weitlaner is placed superiorly and inferiorly. While separating the muscle and soft tissue, one needs to be extremely careful to avoid injury to the vertebral artery. The pulsations of the vertebral artery can be seen and felt. Any brisk bleeding from the venous plexuses can be controlled with Gelfoam, and it is a warning sign that the vertebral artery is nearby. We prefer a small craniotomy (5 cm). The foramen magnum rim is removed with either a highspeed drill or a Kerrison punch. The dural arch of C1 is excised. The dura is opened in a Y-shaped manner. At this stage, the microscope is brought into the field. The arachnoid is opened separately. The tonsils are usually tongue-like and smooth, very often pulling down the posterior inferior cerebellar artery along with them into the spinal canal. Care has to be taken to avoid injury to the artery while dissecting the tonsils from the arachnoid adhesions (Fig. 6.7). We routinely shrink the tonsils with bipolar coagulation. A fascia lata graft is used for duraplasty, and the dura is closed in a watertight manner.

♦ Outcome

In the vast majority of centers, the procedure has yielded good results. Postoperative MRI established the efficacy of this method in providing patent CSF pathways around the craniocervical junction.^65,66 Park et al³³ found improvement in all their patients over long-term follow-up. Klekamp et al⁶⁷ reported an 87% decrease in the size of the syrinx in their series of 131 patients, whereas Garcia-Uria et al⁶⁸ found 50% improvement and 75% stabilization over a follow-up period of 5 to 10 years, respectively. Fischer⁶⁹ reported resolution of the syrinx in 93% of cases in a literature survey. Anderson et al⁶² reported postoperative improvement in BAEPs following craniocervical decompression. Not only did syringomyelia and symptoms of hindbrain compression improve, but also scoliosis, which is seen with long segment syringomyelia, responded to posterior cranial fossa (PCF) decompressive surgery.^70,71

♦ Controversies

There are several controversies regarding intradural procedures such as dissection of the arachnoid overlying the tonsils, shrinkage of the tonsils by bipolar coagulation, and subpial resection of the tonsils. A recent study found that tonsillar management has no significant effect on improvement of syringomyelia.⁷²

In some studies the results are good without any Intradural manipulation.^73–75 Genitori et al⁷⁴ studied the role of posterior fossa bony decompression without dural opening in the management of symptomatic children affected with Chiari type I malformation. With this approach they achieved improvement in symptomatology in a high percentage of cases (97.2%). A similar strategy was adopted by James et al.⁷⁶ Zamel et al⁶¹ described their experience with neurophysiologic intraoperative monitoring of BAEPs during posterior fossa decompression surgery for the management of Chiari I malformation. Their findings suggest that posterior fossa decompression with bone removal alone significantly improves conduction time in most pediatric patients with Chiari I malformation. The use of duraplasty facilitated a small improvement in conduction time in only 20% of the patients.⁶¹

Fig. 6.7 Intraoperative photograph shows arachnoid adhesions and bands around the tonsillar ectopia. A low-lying posterior inferior cerebellar artery is also seen overlying the spinal cord.

In another study, the authors adopted a dura-splitting decompression for pediatric Chiari I malformation.⁷³ The early clinical results were good, and this technique significantly reduced resource use.⁷³

Others have described foramen magnum decompression with removal of the outer layer of the dura as a treatment option for syringomyelia occurring with Chiari I malformation.^75,77

Recently, Sindou and Gimbert⁷⁸ described decompression for Chiari type I malformation by extreme lateral foramen magnum opening and expansile duraplasty with arachnoid preservation. They claim that this type of craniocervical decompression achieved the best results with minimal complications and side effects.

This is a difficult entity for decision making as well as for operative detail. There is no uniform consensus on the management of these lesions. The problem lies in choosing the appropriate patients for surgery and the type and extent of the surgical intervention. In patients presenting with symptomatic Chiari II malformation due to medullary dysfunction, early surgical intervention may be life sustaining.79

Ventriculoperitoneal Shunting

According to Tubbs et al,⁷⁹ a properly functioning ventricular shunt can obviate the need for hindbrain decompression. The difficulty lies with the evaluation of shunt function in these patients. In the presence of a shunt, progressive hydrosyringomyelia occurs due to shunt malfunction, unless proven otherwise.⁷⁹ Periodic evaluations for brainstem malfunction need to be performed, especially by otolaryngologists and internists focusing on respiratory function, swallowing, and speech.

Posterior Fossa Decompression

In cases where progressive brainstem compression is clinically evident, posterior fossa decompression is indicated. One needs to keep in mind that the foramen magnum is wide in these patients and the torcular and the transverse sinus are low lying. Extensive bone removal may not be necessary and is dangerous if the sinuses are entered inadvertently.⁷⁹ Patients may need postoperative cervical spine evaluation, because there is a possibility of delayed cervical instability.⁸⁰ After midline durotomy, the arachnoid adhesions need to be lysed, and cerebellar vermis is released from compressing the cervicomedullary junction. Some patients require coagulation of excessive vermis that might be obstructing CSF flow. The foramen Magendie is carefully dissected free of the adhesions, and the CSF pathways are opened up.

Outcome

Posterior fossa decompression relieves the symptoms of syringomyelia in more than 75% of the patients and is preferred over syrinx shunting.⁸¹ If the syrinx persists and the patient is symptomatic, then a syringosubarachnoid, syringopleural, or syringoperitoneal shunt could be considered.⁷⁹ Intrauterine repair of a myelomeningocele may reduce the extent of hindbrain abnormalities.⁸² The intrauterine repair of a myelomeningocele might reduce the fluid loss and thus the hindbrain herniation through the foramen magnum.

Chiari III Malformations

Much of the evidence comes from a relatively small number of series with very few cases. According to a recently published literature review, there are only 34 cases with Chiari III malformation.⁵⁷ These patients may require VP shunting for hydrocephalus, repair of an encephalocele, and management of other congenital anomalies. If there is a hydrocephalus, VP shunting should be performed first.⁵⁷ Snyder et al⁵⁶ managed a patient with Chiari III malformation with an initial VP shunt. The shunt allowed the brainstem and the cerebellum to regress into the cervical spinal canal. A delayed closure of the cervical encephalocele was performed at 30 months of age. In another study, hydrocephalus was noted in seven of eight patients with Chiari III malformation, and five of them with initial hydrocephalus underwent shunting followed by closure of the encephalocele.⁵⁷

The repair of the malformation follows the same principles that are applicable to open neural tube defects. The amount of neural tissue in the encephalocele varies. Great care is required to preserve neurologic function. MRI enables the surgeon to identify the amount of neural tissue in the encephalocele and the position of the brainstem and venous anomalies.⁵⁰ Delayed treatment of hydrocephalus, initial severity of neurologic deficits, the amount of brain tissue within the excised encephalocele, the presence of intermittent apnea, and surgery in the newborn period were established as prognostic factors for a poor outcome.⁵⁷

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